Serial data receiver with decision feedback equalization

ABSTRACT

An apparatus includes first and second receiver circuits and a decision circuit. The first receiver circuit is configured to generate a first data symbol from a particular input data symbol of a plurality of input data symbols included in an input signal. The second receiver circuit is configured to generate a second data symbol from the particular input data symbol. The decision circuit is configured to select, using respective values of one or more previous output data symbols, either the first or second data symbol as a current output data symbol. In response to a change in value between successive input data symbols, the first and second receiver circuits are configured to generate the first and second data symbols with respective data valid windows with different durations.

BACKGROUND Technical Field

Embodiments described herein are related to the field of integratedcircuits, and more particularly to serial communication circuits in anintegrated circuit.

Description of the Related Art

A computer system or integrated circuit, such as a system-on-a-chip(SoC), may include one or more interfaces for communication with otherICs. For example, an SoC may include a double-data rate (DDR) interfacefor communicating with a dynamic random-access memory (DRAM) module. Asaccess times may directly impact performance of an SoC, there is adesire to transfer data between the SoC and the DRAM module as quicklyas possible. DDR interfaces may, therefore, be designed for high datatransfer frequencies.

In combination with a desire for high performance computer systems,prevalence of mobile computing devices drives a desire for lower powercomputing systems, including low power DDR interfaces that operate atlower voltage levels. In order to receive high-speed/low voltagesignals, a differential amplifier may be employed. A differentialamplifier used to receive signals from a DDR DRAM module may alsoutilize a bias voltage generator as well as a reference voltagegenerator. Such circuits, however, may consume an undesirable amount ofpower, resulting in reduced battery life in a mobile computing device.

SUMMARY OF THE EMBODIMENTS

Broadly speaking, a system, an apparatus, and a method are contemplatedin which the apparatus includes first and second receiver circuits and adecision circuit. The first receiver circuit may be configured togenerate a first data symbol from a particular input data symbol of aplurality of input data symbols included in an input signal. The secondreceiver circuit may be configured to generate a second data symbol fromthe particular input data symbol. The decision circuit may be configuredto select, using respective values of one or more previous output datasymbols, either the first or second data symbol as a current output datasymbol. In response to a change in value between successive input datasymbols, the first and second receiver circuits may be configured togenerate the first and second data symbols with respective data validwindows with different durations.

In a further example, to generate the first data symbol, the firstreceiver circuit may have a first input voltage trip point that is lowerthan a second input voltage trip point of the second receiver circuit.In one example, to set the first input voltage trip point, the firstreceiver circuit includes a first plurality of transconductance devicescoupled between a first output node and a ground reference node. Inanother example, to set the second input voltage trip point, the secondreceiver circuit may include a second plurality of transconductancedevices coupled between a second output node and a power signal.

In an embodiment, the first and second receiver circuits may be furtherconfigured to enable a respective one of the first and secondpluralities of transconductance devices based on a control signal suchthat, when the control signal is asserted, the first input voltage trippoint is decreased and the second input voltage trip point is increased.In one example, the first and second receiver circuits may be furtherconfigured to generate the first and second data symbols such that adata valid window is longer for the first data symbol than for thesecond data symbol when the input signal transitions from a logic low toa logic high, and the data valid window is longer for the second datasymbol than for the first data symbol when the input signal transitionsfrom a logic high to a logic low.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description makes reference to the accompanyingdrawings, which are now briefly described.

FIG. 1 illustrates a block diagram of an embodiment of a receiversystem.

FIG. 2 shows two circuit diagram of embodiments of inverting stages usedin a receiver circuit.

FIG. 3 depicts two charts of waveforms associated with an embodiment ofa receiver system that utilizes a single input voltage trip point.

FIG. 4 illustrates two charts of waveforms associated with an embodimentof a receiver system that utilizes two input voltage trip points.

FIG. 5 depicts a block diagram of a computing system that utilizes thereceiver system shown in FIG. 1.

FIG. 6 illustrates a flow diagram of an embodiment of a method foroperating a receiver system with two receiver circuits.

FIG. 7 shows a flow diagram of an embodiment of a method for setting aninput voltage trip point for each of two receiver circuits in a receiversystem.

FIG. 8 depicts a block diagram of an embodiment of a computer systemthat includes a receiver system.

FIG. 9 illustrates a block diagram depicting an examplecomputer-readable medium, according to some embodiments.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the disclosure to theparticular form illustrated, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present disclosure as defined by the appendedclaims. As used throughout this application, the word “may” is used in apermissive sense (i.e., meaning having the potential to), rather thanthe mandatory sense (i.e., meaning must). Similarly, the words“include,” “including,” and “includes” mean including, but not limitedto.

Various units, circuits, or other components may be described as“configured to” perform a task or tasks. In such contexts, “configuredto” is a broad recitation of structure generally meaning “havingcircuitry that” performs the task or tasks during operation. As such,the unit/circuit/component can be configured to perform the task evenwhen the unit/circuit/component is not currently on. In general, thecircuitry that forms the structure corresponding to “configured to” mayinclude hardware circuits. Similarly, various units/circuits/componentsmay be described as performing a task or tasks, for convenience in thedescription. Such descriptions should be interpreted as including thephrase “configured to.” Reciting a unit/circuit/component that isconfigured to perform one or more tasks is expressly intended not toinvoke 35 U.S.C. § 112, paragraph (f) interpretation for thatunit/circuit/component. More generally, the recitation of any element isexpressly intended not to invoke 35 U.S.C. § 112, paragraph (f)interpretation for that element unless the language “means for” or “stepfor” is specifically recited.

As used herein, the term “based on” is used to describe one or morefactors that affect a determination. This term does not foreclose thepossibility that additional factors may affect the determination. Thatis, a determination may be solely based on specified factors or based onthe specified factors as well as other, unspecified factors. Considerthe phrase “determine A based on B.” This phrase specifies that B is afactor that is used to determine A or that affects the determination ofA. This phrase does not foreclose that the determination of A may alsobe based on some other factor, such as C. This phrase is also intendedto cover an embodiment in which A is determined based solely on B. Thephrase “based on” is thus synonymous with the phrase “based at least inpart on.”

DETAILED DESCRIPTION OF EMBODIMENTS

High-speed serial communication circuits may be utilized in anintegrated circuit (IC) for a variety of interfaces, such as, Ethernet,universal serial bus (USB), serial AT attachment (SATA), and double-datarate (DDR) interfaces. In some designs, multiple serial communicationcircuits may be utilized in parallel to further increase data transferspeeds by sending one bit of a data word (referred to herein as a datasymbol) via each serial communication circuit.

To improve data rates for high-speed serial data communication across acommunication channel, decision feedback equalization (DFE) may beimplemented in serial receiver circuits to attenuate lasting effectsfrom previously received data symbols as well as effects from thephysical properties of the communication channel itself. Variouscharacteristics of a connection between a transmitter circuit and areceiver circuit, for example, a length of a wire, an impedance of thewire, electromagnetic coupling to other nearby wires, and the like, maydetermine an amount of influence previous data symbols have on a currentdata symbol. For example, a logic high data symbol, represented on awire as a high voltage level, may cause a subsequent logic low datasymbol, represented on the wire by a low voltage level, to have a highervoltage level than if the first data symbol was a logic low. In variouscases, the influence of the voltage level of a given data symbol maypersist on the wire one, two, or more subsequent data symbols.

To mitigate bit errors that may occur due to the physicalcharacteristics of a communication channel, some communication systemsmay use differential amplifiers to receive an input signal and generatea stream of data symbols based on the input signal. In addition, thesecommunication systems may also utilize a bias voltage generator as wellas a reference voltage generator in combination with the differentialamplifiers. These differential amplifiers may consume a relatively highamount of power, and may have increasing bit error rates as data ratesincrease. A reduced-power option with improved bit error rates is,therefore, desired.

Embodiments of apparatus and methods are presented for receiving aserial data input signal. The apparatus includes at least two receivercircuits. A first receiver circuit is configured to extend a data validwindow of first data symbol in response to detecting a particular stateon the input signal. A second receiver circuit is configured to extend adata valid window of second data symbol in response to detecting adifferent state on the input signal. The data symbols from the first andsecond receivers are received by a decision circuit which selects one ofthe two received data symbols based on at least one previouslydetermined output data symbols, and uses the selected data symbol todetermine a next output data symbol. Use of such an apparatus mayimprove data valid windows sampled from a received input signal, forexample, by increasing a width of an effective data eye of the receivedinput signal. These improved data windows may be capable of supportingfaster data transfer rates and/or lower voltage levels on input signalwith little or no increase in bit error rates.

A block diagram for an embodiment of a receiver system is illustrated inFIG. 1. Receiver system 100 may be included in an integrated circuit(IC) as part of a communication interface, for example a memoryinterface such as LPDDR3, LPDDR4, Wide I/O n, and High Bandwidth Memory(HBM). Receiver system 100 may, therefore, represent a receiver channelfor one serial bitstream of a plurality of bitstreams that combined forma data word. In various embodiments, receiver system 100 may be used tocommunicate with an IC in another package, an IC on another die in asame package, or other circuits within a same IC. As illustrated,receiver system includes first receiver circuit 101 and second receivercircuit 103, both coupled to decision circuit 110.

Input signal 120 is received by receiver system 100 and sent to bothfirst receiver circuit 101 and second receiver circuit 103. Firstreceiver circuit 101, as shown, is configured to generate first datasymbols 123 from respective ones of input data symbols 121 included ininput signal 120. In a similar manner, second receiver circuit 103 isconfigured to generate second data symbols 125 from respective ones ofinput data symbols 121. First receiver circuit 101 and second receivercircuit 103 are further configured to generate first data symbols 123and second data symbols 125 such that a data valid window is longer fora given first data symbol than for a corresponding second data symbolwhen input signal 120 transitions from one logic state to another.Similarly, a data valid window is longer for a given second data symbolthan for a corresponding first data symbol when a different logictransition occurs on input signal 120.

As used herein, a “data symbol” refers to a particular voltage level atan input node that represents a respective value for one or more bits ofinformation. In some embodiments, one bit may be represented by avoltage level on a single node such that a voltage level above athreshold voltage level corresponds to a logic high value, or a ‘1’ anda voltage level below the threshold voltage level corresponds to a logiclow value, or a ‘0.’ In other embodiments, a pair of input nodes may beused to receive two input signals to determine a value of a single bit,such as a differential signal. Differential signaling uses differentvoltage levels in a data symbol to determine a data value. For example,when a first voltage level of a first input node is above the thresholdvoltage and a second voltage level of a second input node is below thethreshold voltage, then the bit has a logic high value and vice versa.If the voltage level on both input nodes is above or below the thresholdlevel, then the data is invalid.

Decision circuit 110, is configured to select a particular data symbolfrom either first data symbols 123 or second data symbols 125 as acorresponding one of output data symbols 127. To select a given datasymbol from either first data symbols 123 or second data symbols 125,decision circuit 110 uses respective values of one or more previous datasymbols from output data symbols 127. Use of previously selected datasymbols to select a current data symbol is referred to herein asdecision feedback equalization (DFE). DFE is based on the knowledge thata current voltage level of an input node may be influenced by a previousvoltage level on the input node. As previously described,characteristics of a connection between a transmitter circuit and areceiver circuit may determine an amount of influence one or morepreviously received data symbols have on a current data symbol.

For example, input signals 120 may include input data symbols 121 a, 121b, and 121 c. In this example, input data symbols 121 have one of twologic states, logic high and logic low, each logic state correspondingto a particular state of a particular characteristic of input signal 120(the particular state being, for example, a voltage level or an amountof current). In other embodiments, however, additional logic states maybe included by using additional voltage levels and/or differentialsignaling. When sequential data symbols encode different logic states(or values), a transition of the voltage level on the input signal mayoccur between the sequential data symbols. For example, a first voltagelevel transition occurs between input data symbols 121 a and 121 b, anda second voltage level transition occurs between input data symbols 121b and 121 c. As shown, first receiver circuit 101 generates wider datasymbols for input data symbols 121 a and 121 c, while second receivercircuit generates a wider data symbol for input data symbol 121 b. If,however, several consecutive data symbols have a same value (i.e., notransition occurs before and after a particular data symbol), then bothfirst receiver circuit 101 and second receiver circuit 103 may generaterespective first and second data symbols that are a same length. Validdata windows may be increased when transitions occur on input signal120.

Based on data values for previously received data symbols, decisioncircuit 110 may select a data symbol from first receiver circuit 101(e.g., first data symbol 123 c) if a value for a previous output datasymbol 127 (e.g., output data symbol 127 b) indicates that input datasymbol 121 b will tend to pull input signal 120 to a first voltagelevel. In a similar manner, decision circuit 110 may select a datasymbol from second receiver circuit 103 (e.g., second data symbol 125 b)if a value for a previous output data symbol 127 (e.g., output datasymbol 127 a) indicates that input data symbol 121 a will tend to pullinput signal 120 to a second voltage level.

Put differently, decision circuit 110 may determine that the voltagelevel of the input node may be skewed to the second voltage level, basedon a data value of output data symbol 127 a. In response to thisdetermination, second data symbol 125 b is selected from second datasymbols 125 to compensate for the skew to the second voltage level,thereby increasing a data valid window if input data symbol 121 btransitions. If, however, input data symbol 121 b does not transition,then second data symbol 125 b will remain in a same logic state assecond data symbol 125 a. When a signal transition occurs on inputsignal 120 that may be hindered by effects from previous data symbols,decision circuit 110 is configured to select a data symbol that is validfor a longer time period. A longer data valid window may increase anamount of time for sampling circuits in receiver system 100 to detectthe correct value of the data symbol. Shorter data valid windows,therefore, may result in higher bit error rates as timing for a datastrobe may be more difficult to set with the shorter data valid windows.

As used herein, a “data valid window” refers to an amount of time that acharacteristic of an input signal reaches and remains in a particularstate that corresponds to a particular value of a data symbol. Forexample, if a high voltage level corresponds to a logic high data value,then the data valid window for a given data symbol is an amount of timethat the voltage level of the input signal remains above a thresholdvoltage for detecting a logic high voltage. If a logic high data valueoccurs on three successive data symbols, then the middle data symbol mayhave a data valid window that spans an entire length of the data symbol.In contrast, if the second data symbol has logic low data value whilethe first and third data symbols have logic high data values, then thedata valid window may be reduced by an amount of time that the voltagelevel of the input signal spends in transition between the high voltagelevel and the threshold voltage for detecting a logic low voltage level.

It is noted that receiver system 100 as illustrated in FIG. 1 is merelyan example. The illustration of FIG. 1 has been simplified to highlightfeatures relevant to this disclosure. Various embodiments may includedifferent configurations of the circuit blocks, including additionalcircuit blocks, such as, for example, additional power samplingcircuits.

The receiver system illustrated in FIG. 1 is shown with two receivercircuits. These receiver circuits may be implemented according tovarious design techniques. A particular example of such a design areshown in FIG. 2. As illustrated, embodiments of inverting stages thatmay be used in receiver circuits are shown. Inverting stage 210 includessix transconductance devices, Q201-Q206 and inverter circuits (INV) 227and 229. An input signal is received on input node 221 and an outputsignal is generated on output node 222. Inverting stage 220 alsoincludes six transconductance devices, Q211-Q216, and receives an inputsignal on input node 223 and generates an output signal on output node224. Embodiments of first receiver circuit 101 and second receivercircuit 103 are illustrated that include arrangements of inverting stage210 and inverting stage 220.

In order to adjust durations of valid windows, first and second receivercircuits 101 and 103 may employ various techniques. For example, thesereceiver circuits may employ different trip points. Raising a level of atrip point may increase a length of logic low data valid windows anddecrease a length of logic high data valid windows, and vice versa whenlowering a level of a trip point.

As illustrated, first and second receiver circuits 101 and 103 receiveinput signal 120 and are configured to generate a logic high output whena voltage level of input signal 120 is greater than their respectivetrip points, and generate a logic low when a voltage level of inputsignal 120 is less than their respective trip points. On theirrespective input nodes, first receiver circuit 101 uses a first trippoint level that is lower than a second trip point level used by secondreceiver circuit 103. First receiver circuit 101 uses inverting stage220 as a first inverting stage, followed by inverting stage 210 as asecond inverting stage. Second receiver circuit 103 is the opposite,using inverting stage 210 as a first inverting stage, followed byinverting stage 220 as a second inverting stage. The respective firstand second trip point levels are determined by the respective firstinverting stages, the first trip point level determined by invertingstage 220 and the second trip point level determined by inverting stage210. Additional details of how a trip point level may affect a validdata window are provided below in the descriptions of FIGS. 3 and 4.

Inverting stage 210 generates a voltage on output node 222 with a logicvoltage level that is complementary to the voltage level on input node221. While the voltage level on input node 221 is above the second trippoint, inverting stage 210 generates a logic low voltage level on outputnode 222, and conversely, generates a logic high voltage level on outputnode 222 while the voltage level on input node 221 is below the secondtrip point. To set the second trip point, inverting stage 210, as shown,includes a plurality of p-channel metal-oxide-semiconductor (PMOS)transistors (Q202-Q206) coupled between output node 222 and a groundreference node, and a n-channel metal-oxide-semiconductor (NMOS)transistor (Q201) coupled between output node 222 and a power node.Although NMOS and PMOS transistors are used in the illustratedembodiment, in other embodiments, any suitable type of complementarytransconductance devices may be used.

Q201 and Q202 are coupled to form an inverter circuit. If Q203-Q206 areignored, then the circuit formed by Q201 and Q202 will generate thecomplement of the logic level on input node 221 on output node 222. TheNMOS Q201 conducts an increasing amount of current between output node222 and the ground reference node as the voltage level on input node 221increases towards a threshold voltage of Q201. The PMOS Q202 conducts anincreasing amount of current between the power node and output node 222as the voltage level on input node 221 decreases towards a thresholdvoltage of Q202. If Q201 and Q202 are similarly sized, then the secondtrip point level may be approximately equal to one-half of the powernode voltage level.

The addition of Q204 and Q206 adjust the second trip point to a lowervoltage level by increasing a current between output node 222 and thepower node by increasing a number of current paths. Assuming that Q202,Q204 and Q206 are similarly sized, then the amount of current that isconducted between the power node and output node 222 is tripled for asame voltage level on input node 221. To allow for an adjustable firsttrip point, Q203 and Q205 are added to selectively enable the currentpaths through Q204 and Q206, respectively. Control signals 235 and 236,respectively, determine if Q203 and Q205 are on, resulting in thecorresponding path through Q204 and Q206 to be enabled.

When control signals 235 and 236 are de-asserted (logic low), thecontrol terminals of Q203 and Q205 are driven high by inverter circuits(INV) 227 and 229, respectively. The high logic levels from INVs 227 and229 are above the threshold voltages for Q203 and Q205, disabling themand thereby blocking current flow through Q204 and Q206. Inverting stage210 may be configured for a lowest one of its possible trip points withcontrol signals 235 and 236 de-asserted. Asserting control signal 235results in a logic low being applied to the control gate of Q203 by INV227, thereby turning Q203 on and enabling current to flow through Q204based on the voltage level on input node 221. Inverting stage 210 nowhas two PMOS transistors providing current paths from output node 222 tothe power node while Q201 provides the only current path from outputnode 222 to the ground reference node. The level of the second trippoint is thereby increased due to the increased ability of invertingstage 210 to source current from the power node to output node 222, ascompared the ability to sink current from output node 222 to the groundreference node. Asserting control signal 236 instead of control signal235 may result in a similar level of the second trip point. Assertingboth control signals 235 and 236 provides three paths from the powernode to output node 222, thereby further increasing the level of thesecond trip point.

Inverting stage 220 is similar to inverting stage 210, except that thelogic is reversed, resulting in a decreased first trip point whencontrol signals 235 and/or 236 are asserted. Inverting stage 220 isconfigured to generate a logic voltage level that is complementary to alogic level detected on input node 223. In a similar manner as invertingstage 210, inverting stage 220 generates a logic low voltage level onoutput node 224 when the voltage level on input node 223 is above thefirst trip point. Inverting stage 220 generates a logic high voltagelevel on output node 224 when the voltage level on input node 223 isbelow the first trip point.

To set the first trip point, inverting stage 220, as illustrated,includes a plurality of NMOS transistors (Q212-Q216) coupled betweenoutput node 224 and the ground reference node, and a PMOS transistor(Q211) coupled between output node 224 and the power node. Q211 and Q212are coupled to form an inverter circuit similar to Q201 and Q202. Q213and Q215 are included to adjust the first trip point to a higher voltagelevel by increasing a current between output node 224 and the groundreference node by increasing a number of current paths. Assuming thatQ212, Q213 and Q215 have similar properties, then the amount of currentthat is conducted between output node 224 and the ground reference nodeis tripled for a same voltage level on input node 223.

Q214 and Q216 are added to selectively enable the current paths throughQ213 and Q215, respectively, allowing for the first trip point to beadjusted. Control signals 235 and 236, respectively, determine if Q214and Q216 are on, resulting in the corresponding path through Q213 andQ215 to be enabled. As described above for inverting stage 210, controlsignals 235 and 236 may be selectively asserted to adjust the first trippoint from a highest setting (control signals 235 and 236 bothde-asserted) to a lowest setting (control signals 235 and 236 bothasserted). With one of control signals 235 and 236 asserted, the secondtrip point of inverting stage 210 may have a higher voltage level thanthe first trip point of inverting stage 220.

As illustrated, first receiver circuit 101 and second receiver circuit103 each include one instance of inverting stage 210 and inverting stage220. Since both inverting stages 210 and 220 generate complementedoutputs of their respective inputs, first and second data symbols 123and 125 are generated with logic states that correspond to detectedlogic levels of input signal 120.

In first receiver circuit 101, inverting stage 220 receives input signal120, and generates complementary signal 230 based on determined voltagelevels of input signal 120. Inverting stage 220 sends complementarysignal 230 to inverting stage 210. Inverting stage 210 generates firstdata symbols 123 based on the detected logic level of complementarysignal 230. The lower trip point of inverting stage 220 may enable firstreceiver circuit 101 to detect a rising transition of input signal 120faster than inverting stage 210 can detect the rising transition. Sincecomplementary signal 230 has a falling transition in response to therising transition of input signal 120, the higher trip point ofinverting stage 210 may detect this falling transition faster thaninverting stage 220. These trip point levels may result in firstreceiver circuit 101 generating first data symbols 123 that have longerdata valid windows when input signal 120 transitions from a logic low toa logic high in comparison with transitions from a logic high to a logiclow.

In second receiver circuit 103, inverting stage 210 receives inputsignal 120, and generates complementary signal 232 based on determinedvoltage levels of input signal 120. Complementary signal 232 is sent toinverting stage 220 which generates second data symbols 125. The higherlevel of the second trip point of inverting stage 210 may enable secondreceiver circuit 103 to detect falling transitions of input signal 120faster than inverting stage 220, resulting in second data symbols 125having longer data valid windows when input signal 120 transitions froma logic high to a logic low in comparison with transitions from a logiclow to a logic high.

By adjusting the levels of the trip points of first and second receivercircuits 101 and 103, using circuits such as inverting stage 210 andinverting stage 220, data valid windows for first and second datasymbols 123 and 125 may be adjusted. FIGS. 3 and 4, described below,illustrate how the trip points relate to the data valid windows.

Turning to FIG. 3, two charts that include waveforms associated with anembodiment of a receiver circuit are illustrated. Chart 300 illustrateswaveforms for input signal 120 and data symbols 330 as associated with areceiver circuit, for example, first receiver circuit 101 or secondreceiver circuit 103 in FIGS. 1 and 2. Chart 350 depicts the samewaveforms, except that voltage levels of input signal 120 are shiftedhigher by a DC offset. As discussed above, various properties of acommunication channel between a transmitter circuit and a receivercircuit may affect a voltage level of an input signal being sent overthe communication channel.

Input signal 120, as shown in charts 300 and 350, encodes a serialstream of input data symbols represented by high and low voltage levels.The shapes of input signal 120 are the same in both chart 300 and chart350, except that in chart 350 a DC offset has been increased, causingthe waveform to move slightly upward in respect to a ground referencenode. To generate data symbols 330, a receiver circuit utilizes trippoint 340, such as may be achieved by de-asserting both control signals235 and 236 shown in FIG. 2. The receiver circuit generates data symbols330 on an output node.

At time t0 in chart 300, the voltage level of input signal 120 is belowthe level of trip point 340. In response, data symbols 330 is at a logiclow level. The level of input signal 120 is rising and, at time t1,reaches the level of trip point 340. In response, the receiver circuitbegins to transition data symbols 330 from the logic low level to alogic high level. Between times t1 and t2, the voltage level of datasymbols 330 reaches and then remains at a logic high level. This timeperiod during which data symbols 330 may be successfully detected as alogic high is labeled as high data valid window 360 a.

At time t2, the voltage level of input signal 120 falls back below thevoltage level of trip point 340, causing the receiver circuit totransition data symbols 330 back to the logic low level. Between timest2 and t3, the voltage level of data symbols 330 reaches and thenremains at the logic low level. This time period during which datasymbols 330 can be successfully detected as a logic low is labeled aslow data valid window 362 a. At time t3, the voltage level of inputsignal 120 rises back above the level of trip point 340, resulting inanother rising transition on data symbols 330.

Referring to chart 350, input signal 120 has been shifted to slightlyhigher voltage levels while trip point 340 remains at a same level as inchart 300. As previously stated, the shape of the waveform of inputsignal 120 is the same as in chart 300, the waveform is just shifted toa higher voltage offset. Chart 350 illustrates how this shift in thevoltage level of input signal 120 may impact high and low data validwindows 360 b and 362 b. As in chart 300, the voltage level of inputsignal 120 is below trip point 340 at time t0, resulting in data symbols330 being at the logic low level.

At time t1, the level of input signal 120 rises above trip point 340,causing the receiver circuit to transition data symbols 330 to the logichigh level. It is noted that this transition to the logic high leveloccurs sooner in chart 350 than it does in chart 300. Since the voltagelevel of input signal 120 is shifted higher in chart 350, input signal120 needs a smaller increase in the voltage level to reach trip point340, resulting in trip point 340 being reached sooner. Data symbols 330reaches and remains at the logic high level until the voltage level ofinput signal 120 falls back down to trip point 340, at which point datasymbols 330 begins to transition back to the logic low level. Again, itis noted that this transition point is different than in chart 300.Shifting input signal 120 to a higher voltage offset results in a longerduration to high data valid window 360 b as compared to high data validwindow 360 a in chart 300.

Between times t2 and t3, data symbols 330 reaches and then remains atthe logic low level. In contrast to high data valid window 360 b, lowdata valid window 362 b has a shorter duration than low data validwindow 362 b in chart 300 due to the voltage shift of input signal 120.Data symbols 330 may be sampled using a data strobe asserted at aparticular interval. When high data valid windows and low data validwindows have similar durations, a setting of the data strobe may resultin few bit errors. When durations of the data valid windows are skewedto the high or low data valid windows, then more bit errors may beintroduced, resulting in processing time lost to resending the databeing transferred and/or performing error correction algorithms on themisread data.

Proceeding to FIG. 4, another two charts that include waveformsassociated with an embodiment of a receiver circuit are illustrated.Chart 400 illustrates waveforms for input signal 120, first data symbols123, and second data symbols 125 as associated with, for example,receiver system 100 in FIG. 1. Chart 450 depicts the same waveforms witha similar DC offset occurring on input signal 120 as is shown in FIG. 3.As shown in FIGS. 1 and 2, first data symbols 123 are generated by firstreceiver circuit 101, while second receiver circuit 103 generates seconddata symbols 125.

To generate first data symbols 123, first receiver circuit 101 utilizesfirst trip point 451 that is lower than second trip point 452 utilizedby second receiver circuit 103. By using a lower input voltage trippoint than second receiver circuit 103, first receiver circuit 101 willdetect a rising transition, from a logic low to a logic high, of inputsignal 120 before second receiver circuit 103.

Referring to chart 400, the voltage level of input signal 120 is belowboth trip points 451 and 452 at time t0, resulting in both first datasymbols 123 and second data symbols 125 being at logic low levels. Therising voltage level of input signal 120 reaches the lower voltage oftrip point 451 at time t1 before reaching the higher voltage of secondtrip point 452 at time t2. Accordingly, first data symbols 123transitions to the logic high level at time t1 before second datasymbols 125 transitions to the logic high level, at time t2. As thevoltage level of input signal 120 falls, second trip point 452 isreached at time t3 before reaching first trip point 451 at time t4.Second data symbols 125, therefore, transitions back to the logic lowlevel at time t3 before first data symbols 123 transitions at time t4.The lower level of first trip point 451 results in high data validwindow 460 a for first data symbols 123 being longer than the high datavalid window for second data symbols 125. Decision circuit 110 mayselect high data valid window 460 a of first data symbols 123 as one ofoutput data symbols 127, shown in FIG. 1.

First data symbols 123 remains at the logic low level until input signal120 reaches first trip point 451 at time t5, while second data symbols125 remains at the logic low level longer, until time t6 when inputsignal 120 reaches second trip point 452. Accordingly, the highervoltage level of second trip point 452 results in low data valid window462 a for second data symbols 125 being longer than the low data validwindow for first data symbols 123. Decision circuit 110 may, therefore,select low data valid window 462 a of second data symbols 125 as asubsequent one of output data symbols 127. The waveforms of chart 400demonstrate an example of how first receiver circuit 101 and secondreceiver circuit 103, using different trip points, may generate datasymbols with different lengths based on the same input signal 120.

It is noted that, unlike high data valid window 360 a and low data validwindow 362 a in FIG. 3, the durations of high data valid window 460 aand low data valid window 462 a overlap due to the use of first receivercircuit 101 and second receiver circuit 103. The amount of overlap maybe adjusted based on the settings of first trip point 451 and secondtrip point 452. Decision circuit 110 may be configured to select highdata valid window 460 a before a data strobe triggers a sample of highdata valid window 460 a. Decision circuit 110 may then select low datavalid window 462 a before the data strobe triggers a sample of low datavalid window 462 a. The overlap between high data valid window 460 a andlow data valid window 462 a may provide flexibility in when decisioncircuit 110 switches between the two data windows.

Chart 450 illustrates how a change in the voltage offset of input signal120 impacts the high and low data valid windows. Input signal 120, asreceived, has a DC offset similar to what is illustrated in chart 350 ofFIG. 3. Due to this shift, the voltage level of input signal 120 reachesfirst trip point 451 (at time t1) and second trip point 452 (at time t2)sooner than they were reached in chart 400. First data symbols 123 andsecond data symbols 125, therefore, transition from logic low levels tologic high levels sooner than in chart 400. The voltage offset furthercauses input signal 120 to return back to second trip point 452 (at timet3) and to first trip point 451 (at time t4) later than in chart 400.First and second data symbols 123 and 125 transition from the logic highto the logic low levels at time t3 and t4, respectively. Accordingly,high data valid window 460 b for first data symbols 123 and the highdata valid window for second data symbols 125 are longer than thecorresponding data windows in chart 400.

The low data valid windows, however, are shorter than the correspondingdata windows in chart 400. First data symbols 123 remains at the logiclow level until input signal 120 reaches first trip point 451 at timet5, while second data symbols 125 remain at the logic low level untiltime t6 when input signal 120 reaches second trip point 452. Althoughlow data valid window 462 b in chart 450 is shorter than thecorresponding low data valid window 462 a in chart 400, the low datavalid window 462 b in chart 450 still overlaps with high data validwindow 460 b, and is longer than the corresponding low data valid window362 b in FIG. 3. Low data valid window 462 b, therefore, may still beselected by decision circuit 110 and sampled in response to the datastrobe while reducing a risk of incurring an increase in bit errors.

It is noted that the waveforms illustrated in FIGS. 3 and 4 are merelyexamples for demonstrating the disclosed concepts. In other embodiments,waveforms may exhibit noise caused by signal switching in nearbycircuits, coupled to power supply signals from voltage regulators, orother known noise sources.

Receiver circuits and systems, as shown and describe in FIGS. 1-4 may beused in a variety of applications. For example, high-speed communicationsystems may be used to couple a mass-storage system (e.g., a hard-diskdrive, a solid-state drive, or the like) to a computer system.High-speed communication systems may also be used to couple a computersystem to a networking device such as a WiFi router or Ethernet hub. Oneapplication is depicted in FIG. 5.

Moving now to FIG. 5, a block diagram of an embodiment of a computingsystem that utilizes high-speed communication circuits is shown.Computing system 500 includes processing system 550 coupled to dynamicrandom-access memory (DRAM) module 560 via communication bus 580.Processing system 550 includes processing circuit 530 that accesses datain memory banks 565 a-565 d in DRAM module 560, using transmitter system540 and receiver system 100 to communicate with transceiver 570 in DRAMmodule 560.

DRAM module 560 is a memory system that provides RAM storage for use byprocessing system 550. DRAM module 560 may support any suitable memoryinterface standard, such as LPDDR4, LPDDR4X, and LPDDR5, and the like.DRAM module 560 includes memory banks 565 a-565 d (collectively, memorybanks 565). Each of memory banks 565 includes a particular amount of RAMcells used for storing information for processing system 550, such asprogram instructions and associated data. Access to memory banks 565 isprovided through transceiver 570. Transceiver 570 is configured toreceive memory requests from processing system 550 and fulfill theserequests using memory banks 565.

Communication bus 580, in various embodiments, may include any suitablenumber of communication channels between DRAM module 560 and processingsystem 550. Each channel may further include any suitable number ofwires for sending and receiving commands and data. For example, in oneembodiment, communication bus 580 may be compliant with the LPDDR4 and,therefore include two 16-bit data buses and 6-bit command/address buses.Communication bus 580, in such an embodiment, includes at least 44 wiresfor transferring signals between processing system 550 and DRAM module560.

Processing system 550 is configured to issue memory requests to DRAMmodule 560 to store information and access stored information in memorybanks 565. Processing system 550, in various embodiments, may correspondto an integrated circuit (IC) such as a system-on-chip (SoC) or to acircuit board including a plurality of ICs. In some embodiments,processing system 550 may correspond to a memory interface configured toaccess one or more DRAM modules. Processing system 550 includesprocessing circuit 530 which may correspond to one or more processingcores in processing system 550 capable of issuing memory requests toDRAM module 560.

As illustrated, processing circuit 530 uses transmitter system 540 tosend a memory request to transceiver 570 via communication bus 580 usingoutput signal 545. The memory request is received by transceiver 570 andfulfilled using memory banks 565. If a response is required, e.g.,information is being read from memory banks 565, then transceiver 570returns the requested information via communication bus 580 to receiversystem 100. Receiver system 100 receives the information on firstreceiver circuit 101 and second receiver circuit 103 using input signal120. As disclosed, communication bus 580 may support a standard such asLPDDR4 or LPDDR5 and, therefore, may include a plurality of wires. Thereceived information may be sent by transceiver 570 on a subset of thisplurality of wires, such as a set of 16 wires comprising a 16-bit databus within communication bus 580. To receive information from all 16input signals, receiver system 100 includes sixteen or more sets offirst receiver circuit 101, second receiver circuit 103, and decisioncircuit 110. For clarity, only one set is shown in FIG. 5. Operation ofeach set may conform to the descriptions disclosed above.

Wires included in communication bus 580 may, in some embodiments,include copper traces on one or more circuit boards, pins on one or moreconnectors and sockets, and/or wires in one or more cables. Physicalproperties of these various components that form communication bus 580may differ from wire to wire, resulting in different transmissioncharacteristics between the wires. These differing transmissioncharacteristics may result in each wire having different amounts ofsymbol interference from previously received data symbols on a currentdata symbol being received.

To compensate for the differences between input signals received viadifferent wires, first receiver circuit 101 and second receiver circuit103 may include programmable trip points, as shown in FIG. 2 anddescribed above. Control signals 235 and 236 are selectively asserted bycontrol circuit 515 to set a particular trip point for first receivercircuit 101 and second receiver circuit 103. To determine a setting forthe trip points of first receiver circuit 101 and second receivercircuit 103, control circuit 515 may initiate a training operation todetect symbol interference on at least some wires of communication bus580.

In the example of FIG. 5, a training operation may begin with processingcircuit 530 issuing one or more memory requests through transmittersystem 540. These memory requests cause DRAM module 560 to return aparticular data pattern known by control circuit 515. Based on howaccurately data received by first receiver circuit 101 and secondreceiver circuit 103 matches the known data pattern, control circuit 515may assert or de-assert control signals 235 and 236 until the receiveddata achieves an acceptable level of accuracy to the known data pattern.Once the acceptable accuracy has been achieved, the training operationmay be completed and normal operation of receiver system 100 may occur.In various embodiments, the training operation may be repeatedperiodically or in response to particular events, such as an assertionof a reset signal or a bit error rate reaching a particular threshold.

It is noted that the embodiment of FIG. 5 is merely one example todemonstrate the disclosed concepts. Computing system 500 is not intendedto be limiting and other embodiments utilizing receiver system 100 arecontemplated. For example, communication bus 580 may correspond toaerial interface cable and DRAM module 560 may be replaced with anetwork router.

FIGS. 1-5 illustrate block diagrams and waveforms associated with thedisclosed concepts. Various methods may be employed to operate thesedisclosed circuits. Two such methods are discussed in regards to FIGS. 6and 7.

Proceeding now to FIG. 6, a flow diagram illustrating an embodiment of amethod for operating a receiver system in a computing system is shown.Method 600 may be applied to a receiver system, such as receiver system100 in FIG. 1. Referring collectively to receiver system 100 and theflow diagram in FIG. 6, method 600 begins in block 601.

First and second receiver circuits receive an input signal that includesa plurality of input data symbols (block 602). Input signal 120 isreceived by first receiver circuit 101 and second receiver circuit 103.Input signal 120 includes a plurality of data symbols, such as inputdata symbols 121 a-121 c, each symbol representing a particular datavalue determined by, for example, a voltage level of input signal 120 ata particular point in time. In some embodiments, the voltage level ofinput signal 120 at the particular point in time may be influenced by avoltage level of input signal 120 during a previously received one ormore input data symbols 121.

The first receiver circuit generates a first data symbol based on aparticular logic value of a particular one of the plurality of inputdata symbols, the first data symbol having a first data valid window(block 604). As illustrated, first receiver circuit 101 generates firstdata symbols 123 a-123 c based on voltages levels on input signal 120that correspond to input data symbols 121 a-121 c. Each of first datasymbols 123 has an associated data valid window. A duration of each datavalid window for first data symbols 123 is based on a value of thecorresponding input data symbol 121. As shown, data valid windows forfirst data symbols 123 a and 123 c are longer than the data valid windowfor first data symbol 123 b. First receiver circuit 101 may generate thedifferent durations for first data symbols 123 by using a particularinput voltage trip point that detects one type of voltage transitionearlier than another type of voltage transition.

The second receiver circuit generates a second data symbol based on theparticular logic value of the particular input data symbol, the seconddata symbol having a second data valid window, different than the firstdata valid window (block 606). In a similar manner as for first receivercircuit 101, second receiver circuit 103 generates second data symbols125 a-125 c based on the values of input data symbols 121. Like firstreceiver circuit 101, second receiver circuit 103 generates second datasymbols 125 with different durations based on the data valuescorresponding to each of input data symbols 121. Second receiver circuit103, however, generates second data symbol 125 b with a longer durationthan second data symbols 125 a and 125 c. Second receiver circuit 103generates the different durations for second data symbols 125 by using adifferent input voltage trip point than first receiver circuit 101.

A decision circuit selects either the first or second data symbol as anoutput data symbol (block 608). As disclosed above, voltages associatedwith one or more previously received input data symbols 121 mayinfluence a voltage level of a current input data symbol 121. Thesesinfluences may reduce a data valid window of the current input datasymbol 121. To compensate for the possible reduction, decision circuit110 is configured to select a data symbol from either first receivercircuit 101 or second receiver circuit 103, using data values for thepreviously received input data symbols. Using these previously receivedvalues, decision circuit 110 may select a data symbol with a longer datavalid window. As shown in FIG. 1, decision circuit 110 selects firstdata symbols 123 a and 123 c, along with second data symbols 125 b, togenerate corresponding output data symbols 127 a-127 c. Method 600 mayrepeat for additional input data symbols 121, and end in block 610 oncethere are no further data symbols.

It is noted that the method of FIG. 6 is an example. In otherembodiments, one or more operations may be performed in a differentorder. For example, although shown as occurring in series, operations604 and 606 may be performed in parallel.

In the description of method 600, first and second receiver circuits aredisclosed as using particular trip points to generate data symbols withdifferent data valid windows. In some embodiments, these trip points maybe programmable, for example as part of a training procedure. A methodfor setting the trip points is disclosed below in FIG. 7.

Turning now to FIG. 7, a flow diagram for an embodiment of a method forsetting voltage level trip points in a receiver circuit is depicted.Method 700 may be applied to a receiver circuit, such as first receivercircuit 101 or second receiver circuit 103 in FIG. 2, during a trainingoperation. Referring collectively to first receiver circuit 101,computing system 500 in FIG. 5, and the flow diagram in FIG. 7, method700 begins in block 701.

A control circuit sets a value for the first trip point by enabling afirst number of transconductance devices coupled between a first outputnode and a ground reference node (block 702). Control circuit 515, inFIG. 5, may initiate a training operation for receiver system 100. Aspart of this training operation, trip points may be set for firstreceiver circuit 101 and second receiver circuit 103. To determine aparticular setting, processing circuit 530 sends one or more memoryrequests to DRAM module 560, causing DRAM module 560 to send a knowndata pattern to receiver system 100 using input signal 120. Based on acomparison of values sampled from input signal 120 to the known datapattern, control circuit may selectively assert one or more of controlsignals 235 and 236.

As shown in FIG. 2, control signals 235 and 236 are coupled to controlgates of transistors Q213 and Q215 of inverting stage 220. An assertionof control signal 235 results in Q213 turning on, and allowing currentto flow, based on a voltage level of input signal 120, from output node224 to a ground reference node via Q214. Asserting control signal 236similarly opens a current path from output node 224 to the groundreference node via Q216. When both control signals 235 and 236 arede-asserted, the trip point for inverting stage 220 may be at a highestselectable setting. Asserting both control signals 235 and 236 mayreduce the trip point of inverting stage 220 to a lowest selectablesetting. Asserting either control signal 235 or 236, may reduce the trippoint of inverting stage 220 to a setting in between the lowest andhighest selectable settings.

The control circuit sets a value for the second trip point by enabling asecond number of transconductance devices coupled between a secondoutput node and a power signal (block 704). In a similar manner, controlcircuit 515 selects a trip point for second receiver circuit 103. Asshown in FIG. 2 control signals 235 and 236 are coupled (via INVs 227and 229) to control gates of transistors Q203 and Q205. Control signal235, when asserted, enables current to flow through Q204 to output node222 based on the voltage level of input signal 120. Asserting controlsignal 236 enables current to flow through Q206 to output node 222 basedon the voltage level of input signal 120. When both control signals 235and 236 are de-asserted, the trip point for inverting stage 210 may beat a lowest selectable setting. Asserting both control signals 235 and236 may increase the trip point of inverting stage 210 to a highestselectable setting. Asserting either control signal 235 or 236, mayreduce the trip point of inverting stage 210 to a setting in between thelowest and highest selectable settings. The method ends in block 710.

It is noted that method 700 is merely an example. In other embodiments,the operations may be performed in a different order. For example,operations 702 and 704 may be performed in parallel.

FIGS. 1-7 illustrate apparatus and methods for a receiver system in aprocessing system. Receiver systems, such as those described above, maybe used in a variety of computer systems, such as a desktop computer,laptop computer, smartphone, tablet, wearable device, and the like. Insome embodiments, the circuits described above may be implemented on asystem-on-chip (SoC) or other type of integrated circuit. A blockdiagram illustrating an embodiment of computer system 800 that includesthe disclosed circuits is illustrated in FIG. 8. As shown, computersystem 800 includes processor complex 801, memory circuit 802,input/output circuits 803, clock generation circuit 804,analog/mixed-signal circuits 805, and power management unit 806. Thesefunctional circuits are coupled to each other by communication bus 811.

Processor complex 801, in various embodiments, may be representative ofa general-purpose processor that performs computational operations. Forexample, processor complex 801 may be a central processing unit (CPU)such as a microprocessor, a microcontroller, an application-specificintegrated circuit (ASIC), or a field-programmable gate array (FPGA). Insome embodiments, processor complex 801 may correspond to a specialpurpose processing core, such as a graphics processor, audio processor,or neural processor, while in other embodiments, processor complex 801may correspond to a general-purpose processor configured and/orprogrammed to perform one such function. Processor complex 801, in someembodiments, may include a plurality of general and/or special purposeprocessor cores as well as supporting circuits for managing, e.g., powersignals, clock signals, and memory requests. In addition, processorcomplex 801 may include one or more levels of cache memory to fulfillmemory requests issued by included processor cores.

Memory circuit 802, in the illustrated embodiment, includes one or morememory circuits for storing instructions and data to be utilized withincomputer system 800 by processor complex 801. In various embodiments,memory circuit 802 may include any suitable type of memory such as adynamic random-access memory (DRAM), a static random access memory(SRAM), a read-only memory (ROM), electrically erasable programmableread-only memory (EEPROM), or a non-volatile memory, for example. It isnoted that in the embodiment of computer system 800, a single memorycircuit is depicted. In other embodiments, any suitable number of memorycircuits may be employed. In some embodiments, memory circuit 802 mayinclude a memory controller circuit as well communication circuits foraccessing memory circuits external to computer system 800, such as aDRAM module 560 in FIG. 5. Receiver system 100 may be included as partof such communication circuits.

Input/output circuits 803 may be configured to coordinate data transferbetween computer system 800 and one or more peripheral devices. Suchperipheral devices may include, without limitation, storage devices(e.g., magnetic or optical media-based storage devices including harddrives, tape drives, CD drives, DVD drives, etc.), audio processingsubsystems, or any other suitable type of peripheral devices. In someembodiments, input/output circuits 803 may be configured to implement aversion of Universal Serial Bus (USB) protocol or IEEE 1394 (Firewire®)protocol.

Input/output circuits 803 may also be configured to coordinate datatransfer between computer system 800 and one or more devices (e.g.,other computing systems or integrated circuits) coupled to computersystem 800 via a network. In one embodiment, input/output circuits 803may be configured to perform the data processing necessary to implementan Ethernet (IEEE 802.3) networking standard such as Gigabit Ethernet or10-Gigabit Ethernet, for example, although it is contemplated that anysuitable networking standard may be implemented. In some embodiments,input/output circuits 803 may include one or more instances of receiversystem 100 to support various communication interfaces.

Clock generation circuit 804 may be configured to enable, configure andmanage outputs of one or more clock sources. In various embodiments, theclock sources may be located in analog/mixed-signal circuits 805, withinclock generation circuit 804, in other blocks with computer system 800,or come from a source external to computer system 800, coupled throughone or more I/O pins. In some embodiments, clock generation circuit 804may be capable of enabling and disabling (e.g., gating) a selected clocksource before it is distributed throughout computer system 800. Clockgeneration circuit 804 may include registers for selecting an outputfrequency of a phase-locked loop (PLL), delay-locked loop (DLL),frequency-locked loop (FLL), or other type of circuits capable ofadjusting a frequency, duty cycle, or other properties of a clock ortiming signal.

Analog/mixed-signal circuits 805 may include a variety of circuitsincluding, for example, a crystal oscillator, PLL or FLL, and adigital-to-analog converter (DAC) (all not shown) configured togenerated signals used by computer system 800. In some embodiments,analog/mixed-signal circuits 805 may also include radio frequency (RF)circuits that may be configured for operation with cellular telephonenetworks. Analog/mixed-signal circuits 805 may include one or morecircuits capable of generating a reference voltage at a particularvoltage level, such as a voltage regulator or band-gap voltagereference.

Power management unit 806 may be configured to generate a regulatedvoltage level on a power supply signal for processor complex 801,input/output circuits 803, memory circuit 802, and other circuits incomputer system 800. In various embodiments, power management unit 806may include one or more voltage regulator circuits, such as, e.g., abuck regulator circuit, configured to generate the regulated voltagelevel based on an external power supply (not shown). In some embodimentsany suitable number of regulated voltage levels may be generated.Additionally, power management unit 806 may include various circuits formanaging distribution of one or more power signals to the variouscircuits in computer system 800, including maintaining and adjustingvoltage levels of these power signals. Power management unit 806 mayinclude circuits for monitoring power usage by computer system 800,including determining or estimating power usage by particular circuits.

It is noted that the embodiment illustrated in FIG. 8 includes oneexample of a computer system. A limited number of circuit blocks areillustrated for simplicity. In other embodiments, any suitable numberand combination of circuit blocks may be included. For example, in otherembodiments, security and/or cryptographic circuit blocks may beincluded.

FIG. 9 is a block diagram illustrating an example of a non-transitorycomputer-readable storage medium that stores circuit design information,according to some embodiments. The embodiment of FIG. 9 may be utilizedin a process to design and manufacture integrated circuits, such as, forexample, an IC that includes computer system 800 of FIG. 8. In theillustrated embodiment, semiconductor fabrication system 920 isconfigured to process the design information 915 stored onnon-transitory computer-readable storage medium 910 and fabricateintegrated circuit 930 based on the design information 915.

Non-transitory computer-readable storage medium 910, may comprise any ofvarious appropriate types of memory devices or storage devices.Non-transitory computer-readable storage medium 910 may be aninstallation medium, e.g., a CD-ROM, floppy disks, or tape device; acomputer system memory or random-access memory such as DRAM, DDR RAM,SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memory such as a Flash,magnetic media, e.g., a hard drive, or optical storage; registers, orother similar types of memory elements, etc. Non-transitorycomputer-readable storage medium 910 may include other types ofnon-transitory memory as well or combinations thereof. Non-transitorycomputer-readable storage medium 910 may include two or more memorymediums which may reside in different locations, e.g., in differentcomputer systems that are connected over a network.

Design information 915 may be specified using any of various appropriatecomputer languages, including hardware description languages such as,without limitation: VHDL, Verilog, SystemC, SystemVerilog, RHDL, M,MyHDL, etc. Design information 915 may be usable by semiconductorfabrication system 920 to fabricate at least a portion of integratedcircuit 930. The format of design information 915 may be recognized byat least one semiconductor fabrication system, such as semiconductorfabrication system 920, for example. In some embodiments, designinformation 915 may include a netlist that specifies elements of a celllibrary, as well as their connectivity. One or more cell libraries usedduring logic synthesis of circuits included in integrated circuit 930may also be included in design information 915. Such cell libraries mayinclude information indicative of device or transistor level netlists,mask design data, characterization data, and the like, of cells includedin the cell library.

Integrated circuit 930 may, in various embodiments, include one or morecustom macrocells, such as memories, analog or mixed-signal circuits,and the like. In such cases, design information 915 may includeinformation related to included macrocells. Such information mayinclude, without limitation, schematics capture database, mask designdata, behavioral models, and device or transistor level netlists. Asused herein, mask design data may be formatted according to graphic datasystem (gdsii), or any other suitable format.

Semiconductor fabrication system 920 may include any of variousappropriate elements configured to fabricate integrated circuits. Thismay include, for example, elements for depositing semiconductormaterials (e.g., on a wafer, which may include masking), removingmaterials, altering the shape of deposited materials, modifyingmaterials (e.g., by doping materials or modifying dielectric constantsusing ultraviolet processing), etc. Semiconductor fabrication system 920may also be configured to perform various testing of fabricated circuitsfor correct operation.

In various embodiments, integrated circuit 930 is configured to operateaccording to a circuit design specified by design information 915, whichmay include performing any of the functionality described herein. Forexample, integrated circuit 930 may include any of various elementsshown or described herein. Further, integrated circuit 930 may beconfigured to perform various functions described herein in conjunctionwith other components. Further, the functionality described herein maybe performed by multiple connected integrated circuits.

As used herein, a phrase of the form “design information that specifiesa design of a circuit configured to . . . ” does not imply that thecircuit in question must be fabricated in order for the element to bemet. Rather, this phrase indicates that the design information describesa circuit that, upon being fabricated, will be configured to perform theindicated actions or will include the specified components.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

What is claimed is:
 1. An apparatus, comprising: a first receivercircuit configured to generate a first data symbol from a particularinput data symbol of a plurality of input data symbols included in aninput signal; a second receiver circuit configured to generate a seconddata symbol from the particular input data symbol; and a decisioncircuit configured to select, using respective values of one or moreprevious output data symbols, either the first or second data symbol asa current output data symbol; wherein, in response to a change in valuebetween successive input data symbols, the first and second receivercircuits are configured to generate the first and second data symbolswith respective data valid windows with different durations.
 2. Theapparatus of claim 1, wherein to generate the first data symbol, thefirst receiver circuit has a first input voltage trip point that islower than a second input voltage trip point of the second receivercircuit.
 3. The apparatus of claim 2, wherein to set the first inputvoltage trip point, the first receiver circuit includes a firstplurality of transconductance devices coupled between a first outputnode and a ground reference node.
 4. The apparatus of claim 3, whereinto set the second input voltage trip point, the second receiver circuitincludes a second plurality of transconductance devices coupled betweena second output node and a power signal.
 5. The apparatus of claim 4,wherein the first and second receiver circuits are further configured toenable a respective one of the first and second pluralities oftransconductance devices based on a control signal such that, when thecontrol signal is asserted, the first input voltage trip point isdecreased and the second input voltage trip point is increased.
 6. Theapparatus of claim 1, wherein the first and second receiver circuits arefurther configured to generate the first and second data symbols suchthat: a data valid window is longer for the first data symbol than forthe second data symbol when the input signal transitions from a logiclow to a logic high; and the data valid window is longer for the seconddata symbol than for the first data symbol when the input signaltransitions from a logic high to a logic low.
 7. The apparatus of claim1, wherein to select either the first or second data symbol, thedecision circuit is further configured to select a data symbol with alonger data valid window than the other data symbol.
 8. A method,comprising: receiving, by first and second receiver circuits, an inputsignal that includes a plurality of input data symbols; generating, bythe first receiver circuit, a first data symbol based on a particularlogic value of a particular input data symbol of the plurality of inputdata symbols, the first data symbol having a first data valid window;generating, by the second receiver circuit, a second data symbol basedon the particular logic value of the particular input data symbol, thesecond data symbol having a second data valid window, different than thefirst data valid window; and selecting, by a decision circuit, eitherthe first or second data symbol as an output data symbol.
 9. The methodof claim 8, wherein generating the first data symbol includes comparing,by the first receiver circuit, the input signal to a first input voltagetrip point that is lower than a second input voltage trip point used bythe second receiver circuit.
 10. The method of claim 9, furthercomprising setting a value for the first input voltage trip point byenabling a first number of transconductance devices coupled between afirst output node and a ground reference node.
 11. The method of claim10, further comprising setting a value for the second input voltage trippoint by enabling a second number of transconductance devices coupledbetween a second output node and a power signal.
 12. The method of claim8, wherein selecting either the first or second data symbol as an outputdata symbol includes selecting a respective data symbol that has alonger data valid window than the other data symbol.
 13. The method ofclaim 12, wherein selecting the respective data symbol with the longerdata valid window includes selecting the respective data symbol based onlogic values of one or more previously selected data symbols.
 14. Anapparatus, comprising: a first receiver circuit configured to: generatea logic high on a first signal when a voltage level of an input signalis greater than a first trip point; and generate a logic low on thefirst signal when a voltage level of the input signal is less than thefirst trip point; a second receiver circuit configured to: generate alogic high on a second signal when a voltage level of the input signalis greater than a second trip point that is higher than the first trippoint; and generate a logic low on the second signal when a voltagelevel of the input signal is less than the second trip point; and adecision circuit configured to select either the first or second signalas an output signal.
 15. The apparatus of claim 14, wherein to set thefirst trip point, the first receiver circuit includes a first invertingstage comprising: a plurality of n-channel metal-oxide-semiconductor(NMOS) transistors coupled between a first output node and a groundreference node; and a p-channel metal-oxide-semiconductor (PMOS)transistor coupled between the first output node and a power node. 16.The apparatus of claim 15, wherein the first receiver circuit includes asecond inverting stage comprising: a plurality of PMOS transistorscoupled between a second output node and a power node; and an NMOStransistor coupled between the second output node and a ground referencenode.
 17. The apparatus of claim 15, wherein a control terminal of aparticular NMOS transistor of the plurality of NMOS transistors iscoupled to a control signal such that the first trip point is decreasedwhen the control signal is asserted.
 18. The apparatus of claim 17,further comprising a control circuit configured to select a state of thecontrol signal based on a plurality of voltage levels detected in theinput signal.
 19. The apparatus of claim 14, wherein to set the secondtrip point, the second receiver circuit includes a first inverting stagecomprising: a plurality of p-channel metal-oxide-semiconductor (PMOS)transistors coupled between a first output node and a power node; and ann-channel metal-oxide-semiconductor (NMOS) transistor coupled betweenthe first output node and a ground reference node.
 20. The apparatus ofclaim 19, wherein the second receiver circuit includes a secondinverting stage comprising: a plurality of NMOS transistors coupledbetween a second output node and a ground reference node; and a PMOStransistor coupled between the second output node and a power node.